Quickform grouped element question

Hi all, With the latest beta of Quickform I am trying to get a bunch of checkboxes in the form, each with the same name, and in a group. So I looked at the examples and ended up with this piece of code: -------- $genres = $this->_getGenres();// fetch genres in an array as id => genrename // Creates a checkboxes group using an array of separators $checkboxes = array(); foreach ($genres as $id => $label) { $box = &HTML_QuickForm::createElement('checkbox', null, null, $label); $box->setValue($id); $checkboxes[] = $box; } $form->addGroup($checkboxes, 'genres[]', 'Genres:', '<br />'); ----- But that yields this error, which I am really in the dark about: [error_message_prefix] => QuickForm Error:

                                    [mode] => 4
                                    [level] => 1024
                                    [code] => -5
                                    [message] => unregistered element
                                    [userinfo] => Element 'genres[]' does not exist in HTML_QuickForm::_loadElement()

----- Now what I think is part of the problem is that I am confused by Quickform's API. Can one of you help me out? Regards, Hans